The present invention relates to an apparatus for performing certain floating point arithmetic operations in a data processing system. More particularly, the invention relates to an apparatus simplifying the completion of floating point arithmetic operations by processing the operands to form an early determination of the value of the "sticky bit" which appears in the floating point resultant value.
The use of floating point arithmetic operations in a data processing system has been a common practice practically since the inception of computer technology. The development of floating point arithmetic hardware has taken many forms, usually with the objectives of simplifying the hardware construction, or enhancing the speed of the arithmetic processing operation. The four arithmetic operations of add, subtract, multiply and divide have usually been accomplished by using specialized subsets of processes involving addition and subtraction. For example, multiplication operations have in many cases been performed by repeated addition processes, and division has been accomplished by a process of repeated subtraction. The efforts made to speed up these processing operations have focused on enhancements and simplifications of hardware circuit design, particularly the adder circuit, which ultimately limits the maximum processing speed of all arithmetic operations. In the case of division, efforts have been made to increase the speed of operation by calculating partial quotients, or by simultaneously predicting multiple quotient bits, to reduce the number of addition or subtraction iterations required for the divide calculation.
An American national standard has been developed in order to provide a uniform system of rules for governing the implementation of floating point arithmetic systems. This standard is identified as ANSI/IEEE Standard No. 754-1985, and is incorporated by reference herein. In the design of floating point arithmetic systems and algorithms, it is a principal objective to achieve results which are consistent with this standard, to enable users of such systems and algorithms to achieve conformity in the calculations and solutions to problems even though the problems are solved using different computer systems. The standard specifies basic and extended floating point number formats, arithmetic operations, conversions between integer and floating point formats, conversions between different floating point formats, conversions between basic format floating point numbers and decimal strings, and the handling of certain floating point exceptions.
The typical floating point arithmetic operation may be accomplished in either single precision or double precision format. Each of these formats utilizes a sign, exponent and fraction field, where the respective fields occupy predefined portions of the floating point number. In the case of a 32-bit single precision number the sign field is a single bit occupying the most significant bit position; the exponent field is an 8-bit quantity occupying the next-most significant bit positions; the fraction field occupies the least significant 23-bit positions. In the case of a double precision floating point number the sign field is a single bit occupying the most significant bit position; the exponent field is an 11-bit field occupying the next-most significant bit positions; the fraction field is a 52-bit field occupying the least significant bit positions.
After each floating point answer is developed, it must be normalized and then rounded. When the answer is normalized, the number of leading zeros in the fraction field is counted. This number is then subtracted from the exponent and the fraction is shifted left until a "1" resides in the most significant bit position of the fraction field.
In designing the hardware and logic for performing floating point arithmetic operations in conformance with ANSI/IEEE Standard 754-1985, it is necessary and desirable to incorporate certain additional indicator bits into the floating point hardware operations. These indicator bits are injected into the fraction field of the floating point number, and are used by the arithmetic control logic to indicate when certain conditions exist in the floating point operation. For example, an "implicit" bit I is set to "1" by the arithmetic control logic when the exponent of the floating point number has a nonzero value. The implicit bit I is created at the time a floating point number is loaded into the arithmetic registers, and the implicit bit I occupies the first bit position in the fraction field of the number. In addition, a "guard" bit G is set by the floating point control logic during certain arithmetic operations, as an indicator of how to round. The G bit occupies a position which is one bit less significant than the least significant bit (LSB) of the result before rounding. Finally, a "sticky" bit S is an indicator bit which is set in all floating point arithmetic operations when any bit of lower precision than the guard (G) bit is a "1," as an indicator that the floating point number has lost some precision.
The extra bits in the fraction field are used exclusively for rounding operations, after the result has been normalized. The guard (G) bit is treated as if it is a part of the fraction; it is shifted with the rest of the fraction, and included in all arithmetic. The sticky (S) bit is not shifted with the fraction, but is included in the arithmetic. It acts as a "catcher" for 1's shifted off the right of the fraction; when a 1 is shifted off the right side of the fraction, the S bit will remain a 1 until normalization and rounding are finished.
In a rounding operation following the IEEE convention, there are four modes of rounding which are used, as follows:
1) round to nearest; PA1 2) round to positive infinity; PA1 3) round to negative infinity; PA1 4) round to zero.
The "round to nearest" mode means that the value nearest to the infinitely precise result should be delivered. If the two nearest representable values are equally near, the one with its least significant bit zero shall be delivered. The "round to positive infinity" mode means that the value closest to and not less than the infinitely precise result should be delivered. The "round to negative infinity" mode means that the value closest to and not greater than the infinitely precise result should be delivered. The "round to zero" mode means that the result delivered should be the closest to but not greater in magnitude than the infinitely precise result.
Unfortunately, any arithmetic circuit utilizing an adder for carrying out an addition or subtraction inevitably involves the generation of carry bits which are propagated from least significant bit positions to more significant bit positions, and can in fact be propagated throughout all bit positions during an arithmetic operation. This has the affect of extending the processing time required for completing a calculation, and various design efforts have been made to deal with this problem. For example, U.S. Pat. No. 4,754,422, issued Jun. 28, 1988, discloses a dividing apparatus utilizing three carry-save adders in an effort to produce a plurality of quotient bits during each iteration or cycle of arithmetic operation. U.S. Pat. No. 3,621,218, issued Nov. 16, 1971, discloses a high-speed divider utilizing a single carry-save adder for producing a plurality of quotient bits during each iteration of the arithmetic operation, and a plurality of registers for holding a sequence of partial quotients used in the operation.
U.S. Pat. No. 4,639,887, issued Jan. 27, 1987, discloses an apparatus for decreasing the latency time associated with floating point addition and subtraction. The invention uses duplicate hardware for the calculation of the arithmetic operation on the fraction portion of a floating point number, and then selects a resultant value based upon exponent differences.
In any floating point operation in a data processing system it is desirable to increase the efficiency of one or more of the floating point operations, for an increase in this efficiency translates directly into a proportionate time savings in systems operation. Certain efficiencies are possible in specialized situations, some of which are illustrated in the foregoing prior art disclosures, and it is important to take advantage of these efficiencies, particularly if the special situations may be encountered relatively frequently during the course of data processing operations. For example, floating point arithmetic calculations frequently require a normalize operation when an answer is developed, and a rounding operation if the answer is inexact. However, either or both of these operations may be skipped when certain result conditions exist, thereby saving the time otherwise required for executing these operations. In floating point multiply operations the normalize and rounding steps can be eliminated approximately 50% of the time, depending upon certain operating conditions, and for floating point addition and subtraction operations the normalize and rounding steps can be eliminated about 25% of the time, depending upon operating conditions. By eliminating these steps when conditions suggest that elimination is possible, an overall savings in computer processing time is achieved.
The states of the guard (G), sticky (S), and the least significant bit (LSB), the resultant sign, and the rounding mode are all used to determine whether or not the LSB should be incremented in order to deliver a correctly-rounded fraction result. The state of the sticky (S) bit must usually be known prior to delivering a final result.